Hi,
Roon uses multicast packets to find RAAT devices, some routers aggressively filter this packets which causes device discover issues, but typically this only affects WiFi connected devices. One way to rule out your router is to connect both the Auralic Altair and the computer running Roon to a network switch rather than directly to the router (the switch would then be connected to the router to complete the hookup on to your LAN).
You mentioned port forwarding, are you running more than one subnet on your LAN?
I’m not familiar with the Altair so I can’t offer any specific advice about that, maybe @support or another Altair user on here could help.
In the meantime in order for @support to accurate evaluate this issue, they will need to gather a bit more data from you.
Could you kindly please expand on the details of your setup as seen here. The more specific you can be, the better esp.